This invention relates generally to a process to produce hydroxyacetaldehyde from a pyrolyzed carbohydrate-containing feedstock and, more specifically, to a process to produce hydroxyacetaldehyde from a water soluble fraction of pyrolysis products obtained from a pyrolyzed carbohydrate-containing feedstock such as wood, cellulose, sugars or starches. Aqueous solutions of hydroxyacetaldehyde are useful to color or brown proteinaceous materials and to produce natural flavors when combined with ammonia or amines.
Hydroxyacetaldehyde, HO--CH.sub.2 --CHO, (HAA) is the simplest aldehyde-alcohol or sugar. Fenton and Jackson, J. Chem. Soc., 79:774 (1902) provide some physical characteristics and a synthesis of HAA. In the reported synthesis, an aqueous solution of dihydroxymaleic acid was heated and then distilled under reduced pressure. A viscous fraction that distilled at near 100.degree. C. was collected and found to crystallize after cooling. Analysis of the crystalline material indicated the material was a crystalline dimer of HAA which was soluble in water or hot alcohol but only sparingly soluble in ether.
Subsequent studies suggest HAA may actually exist in solution in at least three different monomeric or dimeric forms and as a mixture of stereoisomers. See, for example, Michelsen and Klaboe, J. of Molecular Structure, 4:293-302 (1969), Collins and George, J. Chem. Soc. (B), 1352-1355 (1971), and Stassinopoulou and Zioudrou, Tetrahedron, 28:1257-1263 (1972).
Currently, commercially available HAA is sold as a crystalline dimer having a melting point of between 80.degree.-90.degree. C. depending on the stereoisomeric composition of the crystalline material.
HAA has also been identified in complex and variable mixtures of chemicals provided by the pyrolysis of carbohydrate-containing feedstocks. A typical carbohydrate pyrolysis mixture includes condensible liquids, non-condensible gases, and solids. The relative proportions of these materials are dependent on both the type of feed stock and the actual conditions of the pyrolysis. Many of the different chemicals actually identified in typical pyrolysis mixtures are present found in only trace or very small amounts. Other chemicals, however, are present in sufficient quantities to provide the possibility of isolating a particular chemical, such as hydroxyacetaldehyde, if economical separation schemes are available.
An efficient pyrolysis process for carbohydrate-containing feedstocks, especially wood, which gives improved overall HAA yields is described by Underwood et al., U.S. Pat. No. 4,876,108. Briefly, a primary pyrolysis liquid is condensed from wood pyrolysis vapors containing a mixture of water soluble and water insoluble organic compounds. A sufficient amount of water is added to the condensed liquid product to allow the separation of the water insoluble compounds from the water soluble compounds by extraction. The extractable water soluble compounds include a number carbonyl compounds such as sugars, ketones, aldehydes and acids. Identifiable compounds include oligosaccharides, cellobiosan, glucose, fructose, glyoxal, methylglyoxal, levoglucosan, 1,6-anhydroglucofuranose, HAA, formic acid, formaldehyde, acetic acid, ethylene glycol, acetol, acetaldehyde, and methanol. The bulk of the water soluble compounds probably originate from cellulose and hemicellulose present in wood. In total, over 300 compounds have been identified in wood pyrolysis mixtures which makes isolating a specific compound a formidable challenge.
According to Scott et al., "Chemicals and Fuels from Biomass Flash Pyrolysis," Renewable Energy Branch, Energy, Mines and Resources Canada, Ottawa, Canada (1988), the recovery of HAA from a water soluble fraction or from a primary pyrolysis liquid produced from wood presents some severe problems. Specifically, HAA is believed to be very reactive, and if heated, especially in basic solutions, rapidly undergoes condensation reactions.
In unsuccessful attempts to isolate HAA from the water soluble fraction of a wood pyrolysis product, Scott et al. evaluated a number of possible isolation methods including: making a volatile or crystalline derivative, steam distillation, adsorption, chromatographic separation, membrane separation, and metal chelation. The formation of HAA derivatives from the water soluble fraction of a wood pyrolysis mixture was not successful. Similarly, steam distillation was reported to be energy inefficient and yield poor mass recovery. Furthermore, HAA is a water miscible solid which is unsuitable for general steam distillation processes because of its high water solubility. Finally, adsorption and chromatographic methods were difficult or not practical because of the number of essentially similar compounds identified in typical pyrolysis mixtures.
Scott et al. concluded that the most promising approach might be selective adsorption of HAA on silicalite (silicalite is a hydrophobic small-pore high-silica content molecular sieve). However, both acetol and acetic acid, present in all the pyrolysis mixtures, are more efficiently adsorbed by silicalite than HAA. Therefore, even this adsorptive process would require significant refinement to be practical. Of the methods reported by Scott et al., the methods were too complex to be economically practiced on a commercial basis or not selective enough to provide HAA of sufficient purity for end-uses in the food or cosmetic industries.
The large number of compounds produced by pyrolyzing wood is a significant hindrance to the practical separation of useful quantities of any particular compound. The heat lability of HAA, in particular, increases the difficulty of its isolation. Other, less complex feedstocks, may produce somewhat less complex pyrolysis mixtures. For example, the fast pyrolysis of a carbohydrate-containing feedstock such as corn syrup provides a high browning water soluble composition that includes an increased percentage of HAA. Pyrolysis of a less complex carbohydrate-containing feedstocks, such as sugars, also gives multicomponent mixtures of water soluble compounds.
Pragmatically, for the separation of hydroxyacetaldehyde it is desirable to form pyrolysis mixtures having as few other compounds as is possible. Therefore, in certain applications it may be advantageous to pyrolyze a carbohydrate-containing feedstock which provides high yields of HAA with a relatively small number of other pyrolysis compounds.
In sum, a need exists for a process to produce HAA from mixtures obtained by pyrolyzing a carbohydrate-containing feedstock which is economical, efficient and amenable to scale-up. An efficient method of isolating HAA would allow for the use of aqueous HAA solutions as a coloring or browning agent in a foodstuff, as a product which may be used to flavor proteinaceous materials or in cosmetics.